Automatic tuning of transmitters



. MaY'ZZ, 1945- v o. B. CUNNINGHAM ET Al. 2,376,667

AUTOMATIC TUNING OE TRANSMITTERS Filed March 29, 1943 2 Sheets-Sheet lmoe/ma ggz m HNR 7' /Y TEN/V14 *L T L nventors Qttorneg AUTOMATIC TNINGOF TRANSMITTERS Patented May 22, 1945 2,376,667 l AUTOMATIC TUNING OFTRANSMITTERS f Oliver B. Cunningham, Haddoniield, Harry Sussman,Pensauken Township, Camden County, and Samuel Gabin, Merchantville, N.J., assignors to Radio Corporation of America, a corporation of DelawareApplication March 29, 1943, Serial No; 480,928 y (cl. 25o-17) Claims.

This invention relates to automatic tuning of radio transmitters andmore particularly to changing the frequency of operation of a multistagetransmitter by means of a single control.

The usual radio transmitter comprises a master oscillator, one or moreamplifiers, and a power amplifier. The output circuit of thepoweramplier is coupled through resonant circuits to an antenna which isresonated at or near the frequency of operation of the transmitter. Thetuned circuits preceding the final power amplier stage may be arrangedfor single control tuning adjustment by means of mechanical gaugingsimilar to that used in broadcast receivers. However, the poweramplifier and antenna circuits cannot be controlled in this mannerbecause of the wide variation in resistance and reactance of the antennawith variations in frequency. For this reason the adjustment of thefrequency of operation of a transmitter is somewhat cumbersome,requiring the separate tuning of at least three different circuits: theexciter or oscillator andamplier circuits, the power amplifier tankcircuit and the antenna circuit. In addition to this the couplingbetween the antenna and the power amplifier must be readjusted for eachchange in the frequency.

It is the principal object of the present invention to provide animproved method of and means for tuning automatically the circuitsfollowing the master oscillator including the power ampler outputcircuit and the antenna of a radio transmitter.

Other objects will become apparent to those skilled in the art uponconsideration of the following description with reference to theaccompanyingdrawings, of which Fig. 1 is a schematic circuit diagram ofa preferred embodiment of the invention, Fig. 2 is a schematic circuitdiagram of a phase detector employed in the circuit of Fig. 1, and Fig.3 is a modified embodiment of the inventiom j Referring to Fig. 1, apower amplifier stage I is coupled through. a'transformer 3 to anantenna circuit including an adjustable loading inductor 5; The primaryof the transformer 3 is tuned by meansofvariable capacitors 'Iv andsconnected in parallel. The secondary circuitfis' tunedby'a variablecapacitor II and the variableinductor 5. The capacitors 1 and II aremechanically ganged, as indicated by the dash line I3, to the tuningcontrol of the driver amplifier and preceding stages of the transmitter.The capacitor 9 is f mechanicallyfconnected to a reversible motorsimilar motor Il. The secondary of the transformer 3 is movable withrespect to the primary to permit variation of the coupling and is me-Cil I5 andthe inductor 5 is similarly connectedto av 55 chanicallyconnected to a reversible motor I9. A phase detector 2I is connected tothe grid and anode circuits of the power amplier I. Theoutput of thephase detector is connected to an amplier 23. The amplifier 23 isarranged to control the energization 'of "the motor I5 fi'om a powersupply, not shown.

A second phase detector 25 is connected to the anode circuit of thepower amplifier I and to a capacitor 2l included in the ground circuitof the secondary of the transformer 3. An amplifier 29 similar to theamplifier 23"is connected to one group of xed contacts 33 of a relay 3|.The movable contacts 35 of the relay 3l are connected to the motor Il. Asecond group of fixed contacts 3l is connected to the power supplycircuit. A rectifier 39 is connected across the capa'citor 2l and to theenergizing coil 4I ofthe relay 3|.

A resistor 43, bypassed by a capacitor 45, is included in the cathodecircuit of the amplifier I. The resistor 43 is connected through aresistor 4l to the input circuit ofv an amplifier 49. The output circuitof the amplifier 49 is connected to the motor I9. A D.C. source 5I isconnected through' a variable resistor 53` and a meter 55 across theresistor 4l.

Referring to Fig. 2, each of the phase detectors 2| and 25 comprise apair'of diodes 51 and 59 connected between a center tapped inductor 6Iand a center tapped resistor 63. The input circut 6l is connected to a,phase inverter 69. The output of the phase inverter 69 is applied acrossthe inductor El. In order to prevent variations in phase shift throughthe phase inverter with variations in frequency, the load resistor 'I.Iis of very low resistance. This may provide a gain through the phaseinverter of less than unity. The diode 59 is connected to the inputcircuit 6l through an adjustable voltage divider 'I3 to permit balancingof the output of the phase inverter. A capacitor 'I5 is provided betweenthe input circuit 6'l and the control 'grid of the phase inverter tube69. The capacitor 15, in conjunction with the resistance of the voltagedivider 13, provides substantially a 90 degree phase shift of the inputto the circuit 61.

The operation of the phase Shifters shown in Figr2 is as follows:

When the voltage applied to the input circuit between the center taps ofthe elements 6I and B3 is positive at the same instant that the anode ofthe diode 51 is positive, and that of the diode 59 is negative, thediode 51 will conduct during one-half of each cycle and the diode 59will not conduct. This causes the upper end of the resistor 63 to becomepositive with respect to the lower end. Conversely when the voltagebetween the center taps or the voltage across the inductor 6| isreversed, the lower end of the resistor 63 becomes positive. When thetwo voltages are 90 degrees out of phase the Voltage across the upperhalf of the resistor 63 is equal vand opposite to that across the lowerhalf so that the resultant output voltage v.is zero. The phase shiftingnetwork in the input circuit of the phase inverter 69 causes the deviceto provide' zero output when the two input voltages are in phase or 180degrees out of phase, and maximum output when the two voltages are 90degrees out of phase, with a polarity depending upon which of the twoinput voltages leads the other.

The operation of the system of Fig. l is as follows:

The power amplifier circuit is tuned approximately to resonance by meansof the capacitor 1 through the mechanical connections I3 to the tuningelements of the preceding stages. The antenna circuit is similarlyrbrought to a rough approximation of its proper adjustment by means ofthe capacitor I|. The phase detector 2| responds to the voltage in thegrid andanode circuits of the tube I, providing direct current outputrelated in its magnitude and in its polarity to deviation of the outputvoltage from a phase 180 degrees different from that of the inputvoltage. This D.C. voltage controls the amplifier 23, which controls thepower to the motor I5. Thus the motor is energized to run in the properdirection to make the anode voltage exactly 180 degrees out of phasewith the grid voltage by adjustment of the capacitor 9. Under thiscondition the plate current and the plate'voltage of the power amplifierare in phase, and the tank circuit is properly tuned.

The phase detector compares' the tank circuit voltage with the voltageacross the capacitor 21. Since at the beginning of the adjustment cyclethere may be insufficient voltage appearing at the capacitor 21, due todrastic mistuning of thev antenna circuit, 'the motor I1 is normallyconnected through the contact 31 of the relay 3| to the power supply.This causes the motorv I1 to run continuously, driving'the inductor 5 tochange the antenna circuit tuning until some voltage does appear acrossthe capacitor V21. When this occurs the relay 'winding 4I is energizedthrough the rectifier 39, transferring the connections of the motor l1to the output circuit of the amplifier 29. The motor I1 is then operatedunder the control of the phase detector 25 to adjust the antenna circuitto resonance.

The` coupling between the primary and secondary of the transformer 3 isadjusted by the motor I9 in response to the output of the amplier 49.The voltage drop across the cathode resistor of the power amplifier I isproportional to the amplifier plate current. The D.C. source 5I providesa current through the resistor 41, causing a voltage drop which opposesthe voltage across the resistor 43. The drop in the resistor 41 isadjusted to a predetermined value corresponding to the rated platevcurrent of the amplifier I by means of the variable resistor 53 and themeter 55. Thus when plate current of the amplifier is greater than theproper value, a positive voltage is applied to the input of theamplifier 49, and when the plate current is less than the rated value, anegative voltage is ap.- plied to the ampliiier 49.

For example, the tuning of the power amplifier plate circuit may becontrolled by Varying either the capacitance, as shown, or theinductance. Magnetic coupling to the antenna circuit is described in theillustrative embodiment of Fig. l, but any coupling circuit with anadjustable mutual impedance may be used. Any known phase detectorcircuit may lbe substituted for that shown in Fig. 2, depending uponparticular design requirements. In the above describedA system, thepower amplifier is tuned approximately by gang connections to theexciter, and the antenna circuit control searches for rough tuning. Itwill be apparent to those skilled in the art that either of thesemethods may be applied to any of the tunable circuits, and that therough tuning may be omitted providing the phase detectors are designedto operate over the necessary range. The rotation of the motor I9 iscontrolled in such a direction as to adjust the` coupling of atransformer 3 to the correct value.

It is to -be understoodl that the above described adjustments are madeconcurrently so thatif thereis any interaction, each is altered slightlyto make the necessary correction until the proper phase and currentrelations are secured. The motors I5, I1 and I9 may be interconnectedwith' the'respective amplifiers 23, 29 and 49 in known manner to preventhuntingf or overshooting and reversal.

For clarity in description independent motors and servo amplifiers havebeen illustrated. However a single motor may be used with clutchconnections to the various tuning elements for sequential adjustment ofseveral stages. Refer'- ring to Fig. 3, a single motor 11 ismechanically connected as indicated by the dashline 19 to a plurality ofelectro-magnetic clutches 8|, 83, 85 and 81. The clutch 8| is connectedto the exciter tuning control, the clutch 83 is connected to the poweramplier output circuit tuning control, the clutch 85 is connected to theantenna coupling control and the clutch 81 is connected to the antennatuning control. Phase detectors 89, 9| and 93 are connected to theexciter power amplifier and antenna circuit respectively, and a D.C.comparison circuit 95 is connected to the power amplifier, as in thearrangement of Fig. "1. The output circuits of the phase detectors 89,9|, 93 and the comparison circuit 95 arecon# nected to a multiplecircuit switch 91 in the input circuit of a servo amplifier 99. A secondmultiple circuit switch I0| is mechanically ganged with the switch 91and connected to the clutches 8|, 83, 85 and 81. A D.C. source`|93 isconnected to the movable contact of the switch IUI. f

As the D.C. source |03 'is connected through the switch IUI to one ofthe magnetic clutches, for example, the clutch 8|, the input circuit ofthe amplifier 99 is connected through the switch 91 to the correspondingcontrol device, in this case the phase detector 89; The motor" will thenoperate in the same manner as the system of Fig. 1 to adjust the tuningof the exciter stage.' Upon completion of the adjustment of 'the excitertuning the switches 91 and I 0| are moved to their next position,engaging the clutch 83 and con'- necting the phase detector 9| to theservoy amplifier. This operation is continued down the line, each clutchbeing engaged in turn until all-the ldjustments vare completed. Theswitches4 vand IUI may be operated manually, or automatically by meansof a stepping magnet |05 constructed similarly to the stepping switchused in automatic.

telephone exchanges. In the system of Fig. 3, the magnet |05 isenergized from the D.C. source |03 through a relay |01. The actuatingcoil of the relay |01 is connected to the output of the servo amplifier11. Ihus as long as there is any output from the amplifier 99 thecontacts of the relay |01 are held open and the magnet |05 isdeenergized. When adjustment of any stage is completed, the output ofthe servo amplifier falls to zero and the relay |01 is deenergized,connectng the sourcei|03 to the magnet |05 and actuating the switches 91and |0| through one step to connect the next following stage of thetransmitter. A limit switch may be operated by the stepping magnet 85 todeenergize the magnet after the adjustment of the last stage iscompleted.

We claim as our invention:

1. In combination with a radio transmitter including a power amplifier,driver amplifiers, a frequency determining oscillator and an antennacircuit, at least one variable reactance element in the plate circuit ofsaid power amplifier, 'a motor arranged to vary the reactance of .saidelement, a source of energy for said motor, control means connectedbetween said source and said motor for controlling said motor inresponse to the diilerence in phase between the input and the output ofsaid power amplifier, at least one variable reactance element and afurther reactance element in said antenna circuit, a second motorarranged to vary said antenna circuit variable reactance element inresponse to the difference in phase between the output voltage vof saidpower ampliiier and the voltage appearing across said further antennacircuit reactance element, and a third motor arranged to vary thecoupling between the output circuit of said power amplifier and saidantenna circuit in response to the deviation of the plate current ofsaid power amplifier from a predetermined value.

2. In a radio transmitter including an output l tube, a power outputcircuit, an antenna circuit coupled to said output circuit, andmechanically operable means for varying the coupling between saidcircuits, a motor connected to said mechanically operable means, andplate current responsive means connected to said output tube forcontrolling the energization of said motor to increase said coupling inresponse to the deviation of the plate current of said tube below apredetermined value, and to decrease said coupling in response todeviation of said plate current above said predetermined value. y

3. In combination with a radio transmitter including a plurality ofadjustable resonant circuit elements, independent means responsive tothe tuning of each of said elements, an electric motor, independentclutch means connected between said motor and each of said adjustableelements, a control amplier connected to energize said motor, multiplecircuit multiple position switch means connected between the inputcircuit of said control amplifier and each of said tuning responsivemeans so as to connect, in each position, said input circuit to arespective one of said tuning responsive means, a second multiplecircuit'multiple position switch connected between a source of energyand each of said clutch means so as to connect, in each position, saidsource of energy to a respective one of said clutch means, and amechanical connection between said switches, whereby upon actuation ofsaid switches said resonant circuit elements are connected in sequenceto said motor while said tuning responsive elements are connected in thesame sequence to said control amplifier.

4. A radio transmitter comprising a plurality of cascaded stages, eachincluding at least one tunable circuit element and means responsive tothe tuning of said element, a control ampliiier including an inputcircuit and an output circuit, a multiple position switch in said inputcircuit for sequentially and selectively connecting said tuningresponsive means to said input circuit, a motor connected to said outputcircuit to be energized thereby in accordance with the input to saidamplifier, selectively operable clutch means connected between saidmotor and each of said adjustable resonant elements, and multipleposition switch meansfor selectively and sequentially engaging saidclutches.

5. The invention as set forth in claim 3 including means for actuatingsaid switches step by step from one position to another, a source ofenergy for said actuating means, and means responsive to output fromsaid control ampliiier to disconnect said actuating means from saidsource of energy, whereby y said switches are actuated only upondeenergization of said motor.

OLIVER B. CUNNINGHAM.

HARRY SUSSMAN.

SAMUEL GUBIN.

